Syringes prefilled with a liquid drug (generally referred to as “prefilled syringes”) are used as medical syringes. The prefilled syringes are advantageous because of their handling ease without the need for transferring the liquid drug into the syringes. Further, transfer of a wrong liquid drug into the syringe is advantageously prevented. Therefore, the prefilled syringes are increasingly used in recent years.
Unlike conventional syringes into which a liquid drug sucked up from a vial or other container is transferred immediately before use, the prefilled syringes are each required to serve as a container which is kept in contact with the liquid drug for a long period of time.
Such a syringe typically includes a syringe barrel, and a gasket fitted in the syringe barrel and slidable in the syringe barrel.
The gasket to be used for the syringe is generally made of a crosslinked rubber. It is known that the crosslinked rubber contains various crosslinking components, and these components and their thermally decomposed products are liable to migrate into the liquid drug when the liquid drug is kept in contact with the gasket. It is also known that these migrating components adversely influence the efficacy and the stability of some liquid drug.
Further, the gasket is required to be smoothly slidable in the syringe barrel when the syringe is used. In general, the gasket made of the crosslinked rubber is poorer in slidability. To cope with this, it is a general practice to apply silicone oil to an inner surface of the syringe barrel. However, it is also known that the silicone oil adversely influences the efficacy and the stability of some liquid drug.
In view of the foregoing, a product of so-called “laminated gasket” including a rubber gasket body and a highly slidable film laminated on a surface of the gasket body is often used for the medical syringe. Since the surface of the rubber gasket body of the laminated gasket is covered with the highly slidable film, it is possible to ensure the slidability even without the use of the silicone oil while preventing the components of the crosslinked rubber from migrating into the liquid drug (see Patent Documents 1 to 3).
In the laminated gasket, however, the film laminated on the surface is not elastic and, therefore, is liable to impair the elasticity of the inside crosslinked rubber. The elasticity of the gasket is an essential requirement for reliable sealing of the liquid drug contained in the syringe barrel. If the gasket has insufficient elasticity, the liquid drug contained in the syringe barrel is liable to leak out of the syringe barrel. Further, the slidability of the gasket inserted in the syringe barrel also requires improvement.
To cope with this problem, Patent Document 1 proposes a gasket laminated with a tetrafluoroethylene resin (PTFE) film by a casting method. However, this production method is a unique and impractical production method, and is liable to reduce the liquid sealability of the gasket because minute irregularities on a mold surface are transferred onto the surface of the lamination film.
Patent Document 2 proposes a laminated gasket which has a plurality of ring projections provided on a sliding surface thereof and having different outer shapes. However, the ring projections are problematic in slidability because of their greater widths.
Patent Document 3 proposes a production method for producing a laminated gasket which includes an inert resin film provided on a gasket body thereof as extending to a middle of a first annular seal portion thereof to be brought into contact with an inner wall of the syringe barrel. This production method allows for production of a smaller size gasket. However, it is difficult to produce a greater size gasket because of difficulty in stamping.